Dry kiln humidifier



p 15, 1964 E. NICHOLS 3,148,955

DRY KILN HUMIDIFIER Filed Nov. 21, 1960 4 Sheets-Sheet 1 Fig 3 Fig. 2

Fig.2

BULB 90 T BULB 42 BULB 90 Fig.

Ervy Nichols IN VEN TOR.

m- 15, 1964 E. NICHOLS 3,148,955

DRY KILN HUMIDIFIER Filed Nov. 21, 1960 4 Sheets-Sheet 2 v Fig.2

E rvy Nichols INVENTOR.

P 15, 1954 E. NICHOLS 3,148,955

DRY KILN HUMIDIFIER Filed NOV. 21, 1960 4 Sheets-Sheet 3 Ervy Nichols INVENTDR.

United States Patent 3,148,955 DRY KILN HUMIDIFIER Ervy Nichols, 4547 Crittenden Drive, Louisville, Ky. Filed Nov. 2-1, 1960, Ser. No. 70,716 1 claim. or. s4-4s This invention relates to dry kilns of the type generally used for the drying of lumber. Specifically, this invention relates to controlling the humidity in a conventional dry kiln. More specifically, this invention relates to a method and means for providing humidity control at specified drying temperatures, such as are used in the dry kilning of hardwood lumber.

The most common method for drying lumber and timber is that commonly known as air seasoning. This method consists of stacking the timber in the open, so that natural circulation of air around the material reduces its moisture content. The time required for the wood to be dried to a given moisture content depends on atmospheric conditions and the size of the timber pieces, as well as the variety of the timber. The equilbrium concentration of water in the wood which may be reached is in the range of 12 to 20 percent, depending upon atmospheric conditions. However, it is difiicult to reach these figures in a reasonable length of time if the wood is greater than one to two inch stock. Furthermore, there is the constant danger of degradation of the timber by decay, checking and splitting.

For such items as furniture, flooring and interior trim in the hardwood line of timber, it is necessary to dry stocks to a moisture content of from about 6 to 8 per cent. To reach these figures and to avoid degradation during drying, carefully controlled artificial drying methods are employed. These methods usually employ drying kilns.

The drying treatment employed to reduce the equilibrium moisture in finished hardwood stock to from 6 to 8 percent, so as to be suitable for furniture, trim, tool handles, gun stocks, etc. involves heating at a temperature which is incrementally raised from about 120 to about 200 F. and in which the relative humidity is maintained constant for each temperature plateau. Humidity is measured by the well known psychometric method and is expressed as wet-and-dry bulb temperatures.

Thus, for example, in a drying schedude recommended by the Forest Products Laboratory of the United Stataes Department of Agriculture Forest Service, the initial temperature is in the range of 130 F. dry bulb and 120 F. wet bulb. The intermediate drying temperature is recommended at 150-180 F. dry bulb and 130 F. wet bulb. The desired finishing or conditioning temperature is in the range of 170l90 F. with a wet bulb depression of 10-15 Now, if the air is very dry, i.e., the relative humidity is allowed to decrease below a certain point, especially during the initial or final stages of drying, excessive evaporation will cause the wood fibers to become stiff and dry. This condition causes unequal stresses in the boards which result in defects such as checking, warping, splitting, etc.

It is an object of this invention to provide a method and means for controlling the wet and dry bulb temperatures at desired temperature levels in the dry kilning of lumber.

Another object of this invention is to provide a method and means for correlating the wet and dry bulb temperature at desired levels in the drying chamber of the kiln.

Still another object of this invention is the provision of a method for raising the wet bulb temperature in the drying chamber of a kiln while maintaining the dry bulb chamber substantially constant.

Still another object of "this invention is a method for injecting liquid water into the gases going to the kiln Patented Sept. 15,, 1964 to increase the moisture content of the gases in the dry chamber thereof and to cool the gases therein.

Other objects will occur to those skilled in the art from a detailed description which follows.

Drying of wood involves several factors. Chief among these is stress. Stress is defined as force 'per unit area. Stress of sufiicient magnitude results in a strain which is a change in dimension and may be expressed in terms of inches per inch of original dimension. Set is a permanent strain. In the dry kilning of wood, the surface tends to come to the equilibrium moisture content of the kiln atmosphere and thus dries below the fiber saturation point. Thus, the outer zones tend to shrink, but are restrained by the interior zones which not having dried as much have not yet begun to shrink. This results in tensile stress in the outer drier zones and a compressive stress for the inner wetter zones. As the drying continues, successive inner zones change from compression to tension. As compressive stress in the interior zones increases be yond proportional limits, compresison set begins to take place. Tension set occurs in the outer zones wherein the fibers were restrained from shrinking results in a dimension larger than would have occurred had the interior zone been of the same moisture content. According'ly, the stress in the outer zone changes from tension to compression as the interior zones dry and shrink in a more nearly normal manner. When the center zone goes from compression to tension, stress reversal is complete.

Up until the time that stress reversal takes place, it is necessary to maintain a relatively high humidity in the kiln atmosphere at a relatively low temperature. This is not too difficult, however, since moisture is being removed from the timber itself and is recycled throughout the apparatus.

Checking and splitting are caused by reduction of the surface moisture content to a value sufiiciently low so as to cause stresses that exceed the maximum tensile strength of the wood perpendicular to the grain. These are tensile strength stresses which tend to pull the wood apart. By maintaining a relatively high humidity and low temperature during the initial stages of drying, checking and split ting can be essentially eliminated. Furthermore, these need not be feared after stress reversal has taken place. Once stress reversal has occurred, the danger of surface checking and splitting, etc. are essentilly removed so that the temperature can be raised considerably and the relative humidity lowered appreciably. Thus, the wood can be dried to completion.

As has been illustrated previously, tensile stress in the outer zones of the timber tends to prevent shrinkage of said timber, and if the layer of tensile stress can be established sufiiciently long to obtain a tensile set, this may amount to a considerable saving in lumber footage. Thus, it is desirable to very carefully control the temperature, and the relative humidity during the first stages of drying in order to prevent checking and splitting of the timber. In this manner, by maintaining relatively high humidity and a relatively low temperature, tensile stress is set up in a deep outer zone, and maintained until tensile set occurs. Thus, shrinkage is lessened. After stress reversal occurs, the temperature may be raised rather drastically and the relative humidity lowered in order to carry the drying operation to essential completion. When the lumber then reaches a moisture concentration approaching that desired in the final step, equalization and conditioning treatment can be instituted. This treatment consists of raising the wet bulb temperature to within about 820 less than the finished dry bulb temperature. Thus, for example, the dry bulb temperature should be in the range of l70200 F. and the wet bulb temperature in the range of from '140"190 F. In an optimum arrangement, the conditioning or equalizing treatment would be begun immediately merely by addition of high pressure steam. As has been indicated, however, addition of high pressure steam will raise both the wet and dry bulb temperatures. If high pressure steam were admitted, the super heat from said steam would be sulficient to quickly raise the dry bulb temperature far above that desired, thus completely upsetting the temperature humidity relationship. After the lumber has reached the desired end moisture concentration, the conditioning treatment should be instituted. This involves treatment of the wood at an equilibrium moisture concentration at the kiln atmosphere of approximately 4% higher than that desired in the finished product. This treatment results in a reverse moisture gradient in the surface zones of the wood. This increases the compressive stress in the outer zone and increases the tensile stress in the interior zone to overcome the original set in each zone. Due to the high temperature and high relative humidity, a certain plasticization takes place and the stresses are relived all the way across the board. Thus, control of relative humidity or wet bulb temperature is exceedingly important during the first stage of dry kilning and the last stage of dry kilning.

Referring now to figures, wherein like numerals designate like parts throughout, FIG. 1 is a plan view of the entire kiln, that is the furnace and instrument room, plus the drying chamber. FIG. 2 is a side elevation and section taken along lines 22 of FIG. 1. FIG. 3 is a vertical section taken along lines 3-3 of FIG. 1.

FIG. 4 is a diagrammatic view of the wet bulb apparatus and the control apparatus for opening and closing the vents and louvers and the apparatus for opening and closing the water valve.

FIG. 5 is a diagrammatic view of the dry bulb apparatus and the control apparatus for the valve leading to the burner.

Referring now to FIG. 1, the furnace room and control room is designated generally by numeral 1. This room is surrounded by walls 2, 3, 4 and 5. The drying chamber is designated generally by numeral 8 and is surrounded by walls 3, 4, 5 and 6. A partition 7 inside of drying chamber 8 encloses fans 9 and 10 which circulate air in the drying chamber, and are periodically reversed in direction every four minutes.

The furnace 11, inside of the furnace room 1 consists of a squirrel type fan 12, burner 13, combustion chamber 14, firebox 15, jacket 16 and jacket space 17. Communicating with the furnace and the jacket space is mixing chamber 18 which leads via duct 19 to ducts 20 and 21 into the drying chamber 8. As is best illustrated in FIGS. 1 and 2, duct 30 also communicates with drying chamber 8. Air is pulled by fan 25 through duct to structure 27 and via duct 30 to space 26 which communicates with jacket space 17. This air mixes with the combustion products in mixing chamber 18 to form a tempered gas mixture. As has been illustrated in FIG. 2, the corner structure 27 contains a tower 28 and a set of louvers 29 in the outer wall of said structure. At the top of the tower, there is a vent designated by numerals 82 and 80 which is operated by the mechanism, designated generally by the numeral 7 8. The operation of this mechanism which will be described in better detail later also is responsive to changes in the wet bulb temperature and also controls the operation of the louver members 29. Other vents (not shown) on the roof of the kiln may be operated by the same mechanism and in the same manner.

As the wet bulb temperature exceeds that desired i.e. when the relative humidity or moisture concentration exceeds the desired limit at a particular temperature, the vent 82 is opened by raising of the cover 80. At the same time louver members 29 are opened as well as the vents in the roof of the kiln. The hot moisture laden air escapes through said open louvers and said open vents in the roof of the kiln and in the roof of the tower and fresh, cool air is simultaneously admitted. Thus, air is brought in by fan 25, and is carried via duct 3% and 30 through chamber 26 and via jacket space 17 to mixing chamber 18. The mixed gases go via ducts 19 to the drying chamber 8 through ducts 2t) and 21. Excess moisture in the atmosphere of the kiln is expelled through the open vents in the roof of the tower, and the roof of the kiln thus quickly lowering the Wet bulb temperature to that desired. At this point the vents and the louver members are closed. Booster fan 31 is utilized to increase the circulation of gases through ducts 19, 20 and 21, to the drying chamber.

While the kiln disclosed is essentially a closed system, when the vents and louvers are closed, it will be appreciated that there is some leakage in the structure around the doors and the closed vents and louvers. Consequently, air blown into the furnace by fan 12. from the outside, does not build up pressure in the kiln but displaces tempered gases which leak through the cracks and crevices of the kiln structure.

The humidification means is best illustrated in FIG. 2, and consists of main water valve 70, which is controlled by the switch indicated by SW and individual valves 130, 131, 132, 133 and 134. Leading from each of these valves are lines which lead to individual spray head members located in the mixing chamber 18 and in the duct work 19. Thus, for example, leading from and controlled by valve 131) is line which leads to spray members 149. Leading from and controlled by valve 131 is line 137.

Leading from and controlled by valve 132 is line 136 which leads to and controls spray heads 141. Leading from and controlled by valve 133 is line 138 which leads to spray heads 143 and leading from valve 134 is line 139 which controls spray heads 144. Water valve 70 is of the solenoid type and is controlled by the switch to either open or close the water lines completely. Valves 130 through 134 may be either manually or automatically controlled. I have obtained good results with manual controls. A series of valves may be opened since a particular wet and dry bulb temperature is normally required for at least 18 hours or more. In the mixing chamber 18, are bafiles designated by numerals and 160. These bafiles tend to slow down the circulation of gases through the ductwork and to provide wetting surfaces for the contacting of the gases with the liquid water supplied from the spray nozzles aforesaid. In the bottom of mixing chamber 18 is a depression 23 in which water from spray nozzles is collected and drained via drain pipe 24 out of the kiln.

The wet bulb thermometer is designated by numeral 42 and the dry bulb thermometer is designated by 90. The instrument panel is designated generally by numeral 40. The wet bulb temperature control apparatus is illustrated in FIG. 4 and consists of the wet bulb thermometer indicated by numeral 42, a Bourdon type helical spring actuated by said wet bulb thermometer designated by numeral 43 which is attached in turn to arm 44 to pen arm 45. The pen arm is counter-balanced by weight 47 and pivots on pivot support member 46. Attached to pen arm 45 is horizontal link 48 which transmits its movement to pen valve arm 50. The pen valve arm 50 is pivotably fixed at 52 and actuates pin 62 of the needle valve by means of the set screw 53 which holds the said pin in place against the fluid pressure in the valve chamber 63.

The direct set control by which the desired humidity is set is an adjusting nut 58, connected by arm 59 to arm 55 which sets the pointer 54 for the desired thermostatic conditions in the kiln chamber. On the arm. 55 is a slot in which a stud 56 fixed on the arm 51 moves. To stud S6 is fastened a coil spring 57 by which the arm 50, carrying the said set screw 53 is held against pin 62. Arm 51 is pivotably fixed and may move freely the length of the slot in arm 55. This limits the distance that arm 51 may move before the pin 62 of needle valve will be actuated. Inthe side of the needle valve is provided an air leak which is controlled by set screw 67. The fluid pressure is obtained from air reservoir 60 Via supply line 65. Line 73 communicates directly with diaphragm valve 81, the stem of which 74 raises the lever 77. Lever 77 is fixed to a rock shaft 76 which is journaled on the roof of the tower 28 in bearings 75. Rock shaft 76 actuates the cover lever 78 from which is suspended the cover 80 of vent 82. A weight 79 is suspended from lever 77 so as to impose greater resistance to the operation of the pressure actuated diaphragm valve '81. Since the weight is adjustable, the amount of resistance imposed upon the pressure actuated diaphragm valve 81 may be varied. This will determine the time lapse between the operation of the vent and the operation of the switch running from line 69.

Line 69 runs from chamber 64 of said needle valve and the pressure from said line operates the switch indicated by the letters SW. This switch is of the make or break pneumatically operated type and closes the solenoid valve 70 which controls water line 71.

Now when the humidity rises -in the kiln chamber, the helical spring of the wet 'bulb thermometer will be actuated moving the pen arm recording such change on chart 4'1, and simultaneouslyopening the needle valve-63. This pneumatic pressure immediately breaks the circuit thus deenergizing the solenoid and allowing the plunger to fall and close the water line 71. If the humidity continues to rise, the diaphragm valve 81 which controls the roof vent and the louvers will be operated. The time interval between the opening of the vent and the louvers after the closing of the water supply valve is accomplished by adjusting the weight 79 as indicated above. I have had good results when utilizing an air reservoir containing about 20 lbs. pressure of kicking off the solenoid at about 3 lbs. pressure and opening the vent at about lbs. pressure.

Referring now to FIG. 5, numeral 90 designates the dry bulb. Numeral 91 designates the helical spring. Numeral 92 designates the arm connected to said helical spring reflecting the variations in dry bulb temperature. Arm 92 is connected to pen arm 93 which contains counter-balance weight 95 and is connected at pivot point 94. Pen arm 93 pivots on fixed pivot support member 161. Horizontal arm 97 connected to pen arm 93 conmeets with pen valve arm 98. Pen valve arm is pivotably tixed at 99 and may be adjusted by means of set screw 100.

Set screw 100 holds the pin 112, of needle valve 113 in place against the fluid pressure therein.

The direct set control by which the desired temperature is set in an adjusting nut, 109 connected by arm 110 to arm 105 which sets the pointer 103 for the desired dry bulb temperature in the kiln chamber. Contained on arm 105 is slot 106 in which a stud 107 fixed on arm 101 moves. To the stud 107, is fastened a small coil spring 108 by which the arm 98 carrying the set screw 100 is held against the pin 112 of the needle valve 113. The arm 101 is pivotably fixed at point 102 and may move freely the length of the slot 106 in arm 105. Needle valve 113 contains an air leak provided as an escape for pressure in lines 119 and which is controlled by set screw 118. Pressure from air reservoir 60 travels via line 115 through needle valve 113 and line 119 to diaphragm valve 120. As the dry bulb temperature in the kiln exceeds that at which it is set, the stem of diaphragm valve 120 is depressed by the pneumatic pressure to close the orifice in valve housing 121 to the extent that only a pilot flame is left burning in the burner 13. While the control means indicated described above are satisfactory, it will be understood that other control means for operation of the gas valve in response to the dry bulb thermometer and the vents and water valve in response to the wet bulb thermometer can be utilized. Thus, for example a pneumatic control device or an electrical vari able contact control device, such as those disclosed by .Cobb in US. Patent 2,275,042 maybe utilized, and the specific choice of the control device may be left to the discretion of the person designing the kiln.

Operation of the Kiln dry kilning for the variety of beech lumber is as follows:

Dry Bulb Wet -Bu1b Relative Days (24 Temperature Temperature Humidity, hours) percent For purposes our consideration, let us now assume that the wood is essentially dry to its equilibrium concentration of say 8%. This is after 2-3 days of dry kilning, with a dry bulb temperature of 180 and a Wet bulb temperature of 130. It is now desired toraise the wet bulb temperature to 160 from 130 while maintaining the dry bulb temperature at 180. Since there is no high pressure steam available, steam will have to be generated by injection of liquid water into the combustion products. Operation of the burner raises the dry bulb temperature. The apparatus is set for an increase "of wet bulb temperature to 160 F. Valves 131 through 134 are opened. Since the kiln has been operating at 180 F. dry bulb for several days and there is no appreciable heat load, the burner is of pilot size. Immediately, the wet bulb control apparatus is actuated so that the switch is closed to open valve 70. The vent mechanism closes all the vents and lovers. Liquid Water is sprayed from sprayheads 144, so that gases which are drawn from the drying chamber 8 through ducts 30 and 30 via fan 25 through conduit 26 and jacket 17 into mixing chamber 18; are cooled by contact with the water and conducted back into the drying chamber 8 through ducts 20 and 21. These gases are circulated in the kiln until the dry bulb temperature is lowered to such an extent that the dry bulb apparatus actuates the diaphram valve on gas line 122 to completely open said line and the furnace is operating at full capacity. The temperature of the gases emerging from the fire box is about 3000 F. and when mixed with the gases recirculated from the kiln produces a tempered gas mixture in mixing chamber 18, having a temperature of about 750 F. This tempered gas mixture as it ascends contacts baflles and comes into contact with liquid water droplets and is thus cooled to a temperature of about 230 F. by the time it reaches ducts 20 and 21. Normally the temperature of the gases at this point, when the burner is operating at pilot is about 215 F. Thus, it is seen that the effect of the series of sprays is twofold:

(1) To cool the gases normally recirculating so to gradually lower the dry bulb temperature in the kiln and thus actuate the full operation of the burner by opening of the gas valves 70.

(2) The liquid water after cooling the gases sufficiently to actuate the burner to full heating capacity is evaporated by the tempered gas mixture and furnishes moisture to the kiln atmosphere, thus raising the wet bulb temperature therein. In doing so, however, the gas mixture, even with the burner at full capacity enters the kiln at a slightly more than 15 F. higher temperature than it enters when the gases are recirculated and the burner is at pilot. Thus, the dry bulb temperature is are intended or should be inferred except to be commensurate in scope with the appended claim.

I claim:

In a lumber drying kiln having (1) a drying chamber;

(2) a separate furnace chamber, a thermostatically controlled furnace having a burner responsive to a decrease in temperature to automatically increase its output from pilot to full flame;

(3) a heating duct running from the furnace to the drying chamber and discharging into said drying chamber;

(4) a return duct for return of a portion of the atmosphere of the drying chamber to the furnace;

(5) a dry bulb thermometer in said drying chamber operatively connected to the burner of said furnace for automatically adjusting said burner between pilot and full flame;

(6) a wet bulb thermometer in said drying chamber for measuring the wet bulb temperature; and

(7) an air inlet-outlet means in the kiln responsive to said wet bulb thermometer to withdraw hot moisture-laden air and introduce dry cool air;

that improvement in a direct heat exchange cooling and humidifying means comprising,

(a) a mixing duct located between the furnace and the heating duct and connected to said heating duct; wherein the atmosphere from said return duct is mixed with the products of combustion of said furnace to produce an inlet gas,

(12) a liquid spray apparatus, located inside of said mixing duct, and a source of Water opera tively connected thereto to spray unheated liquid water directly into the inlet gas so that said unheated liquid water is partially converted to water vapor upon mixing with said inlet gas so that the gas mixture is cooled sufliciently by direct heat exchange and by conversion of liquid water to Water vapor to lower the dry bulb temperature in the drying chamber, to cause the dry bulb thermometer to adjust the burner from pilot to full flame, and to maintain the temperature of the so tempered gas mixture at such a level so as to maintain the burner at full flame, without raising the dry bulb temperature, until the wet bulb temperature is raised; and

(0) means to drain excess water from said mixing duct.

References Cited in the file of this patent UNITED STATES PATENTS Cone Mar. 12, Arnold Jan. 10, Guthier Aug. 31, Pierce Nov. 29, Sandback May 5,

OTHER REFERENCES Kiln Drying of Lumber, Kohler and Thelen, published by McGraw-Hill Book Co., Inc., New York, 1926 (page 

